The present invention relates to particle beam apparatus in general and in particular to electron multiplier detectors with high backscattered electron acceptance.
Particle beam apparatus, such as the scanning electron microscope, electron beam lithography equipment, ion beam lithography equipment, and the like are used in a wide range of applications. In principle, particle beam apparatus scans the surface of a specimen with an energetic particle beam. The impact of the particle beam on the surface of the specimen causes a release of electrons. In accordance with accepted nomenclature, electrons released with energies exceeding 50 eV are referred to as backscattered electrons (BSEs) while electrons released with energies of 50 eV or less are referred to as secondary electrons (SEs). For a particle beam of approximately 1000 eV, the released electrons are typically divided equally between BSEs and SEs. It is well known that backscattered electrons contain material contrast information while secondary electrons contain topographical information. Particular applications of particle beam apparatus which require high backscattered electron acceptance include inspection of VLSI devices, viewing the bottoms of deep trenches and contact holes in semiconductor materials, and the like.
Early implementations of electron detectors for detecting backscattered electrons are described in a book entitled "Image Formation in Low Voltage Scanning Electron Microscopy" by L. Reimer in SPIE Optical Engineering Press, Bellingham, Wash., U.S.A., 1993, pp. 31-40 which is incorporated by reference as if it were fully set forth herein. Broadly speaking, the electron detectors include a conversion plate for absorbing BSEs and emitting SEs which are, in turn, detected by an Everhart-Thornley scintillator detector. These electron detectors suffer from limited BSE acceptance because the scintillators are typically maintained at a potential of 5 to 10 kV requiring that the detectors be positioned far from the particle beam to prevent large beam deflections and aberrations.
Later implementations of electron detectors for detecting backscattered electrons from a specimen utilize electron detectors fashioned as a microchannel plate electron multiplier or a solid-state electron multiplier. An apparatus including such an electron detector is disclosed in U.S. Pat. No. 4,933,552 to Lee entitled "Inspection System Utilizing Retarding Field Back Scattered Electron Collection". Here, a particle beam is columnated and directed through a bias plate and annular detector at a specimen held at a negative bias with respect to the plate and the detector. The negative bias is selected so that the incident beam strikes the specimen at the crossover energy so that minimum specimen charging occurs. The negative bias also effects spatial separation between SEs and those BSEs having a large transverse velocity component in the plane of the detector.
This arrangement suffers from two main disadvantages. First, the detector has low acceptance of BSEs with small transverse velocity components which, in the case of a particle beam directed at normal incidence to a specimen, can be a substantial fraction of the total BSE yield. In fact, in this instance, the transverse velocity distribution of the BSEs emitted by the specimen has its peak value at zero transverse velocity. And second, in practice, it is difficult to maintain highly insulating specimens at a uniform negative potential while scanning them with a charged particle beam.
Other implementations utilizing microchannel plate or solid-state electron multipliers are described in an article entitled "Low-profile high efficiency microchannel plate detector system for scanning electron microscopy applications," by Michael T. Postek and William J. Keery in Reviews of Scientific Instruments, vol. 61, no. 6, June, 1990, pp. 1648-1657 which is incorporated by reference as if it were fully set forth herein. Here, the particle beam passes through a beam shielding tube on its way to the specimen for preventing deviation and aberration. Hence, the focal point of the beam is directly under the beam shielding tube. When used as a BSE detector, the surface of the electron multiplier is maintained at a potential of -50 V or less with respect to the specimen which is typically grounded.
This implementation also suffers from low BSE acceptance for a number of reasons. First, the electron multiplier only detects BSEs with energies greater than its own potential. Second, many of the BSEs, referred to hereinbelow as Type A BSEs, fail to be detected because they impinge between the active channels of the electron multiplier. And finally, a smaller number of BSEs, referred to hereinbelow as Type B BSEs, are also not detected because they impinge on the beam shielding tube.
These undetected BSEs cause a considerable deterioration in the quality of images rendered by the electron detector because they constitute a meaningful percentage of the total yield of BSEs. As described in the article entitled "Areal detection efficiency of channel electron multiplier arrays" by J. A. Panitz and J. A. Foesch in Rev. Sci Instrum. Vol 47, No. 1, January 1976, the areal detection efficiency of a microchannel plate electron multiplier is typically only about 60%. Thus, Type A BSEs constitute approximately 40% of the total BSE yield. Type B BSEs are usually less numerous, but they often constitute a large fraction of the BSEs in applications where electron emission by the specimen is highly collimated along the particle beam axis.
There is therefore a need for an electron detector having a high backscattered acceptance without suffering from the aforementioned deficiencies.